This disclosure relates to a unique design for a microphone cavity, which can help to reduce wind noise and can assist in pushing resonant frequencies outside of a frequency band of interest (e.g., the voice/speech frequency band).
FIG. 1 illustrates a known mounting arrangement for a microelectromechanical system (MEMS) microphone 100, as recommended by a manufacturer. The microphone 100 is mounted (surface mount soldered) to a bottom surface 102 of a printed wiring board (PWB) 104. An aperture 106 is provided in the PWB 104 to provide an acoustic path to the microphone 100. A gasket 108 is arranged between a top surface of the PWB 104 and a bottom surface of a product housing 110 to provide an acoustic seal therebetween. The housing 110 is typically a hard, plastic cover to the product incorporating the microphone. An aperture 112 is provided in the housing 110, and is aligned with the aperture 106 in the PWB 104, to extend the acoustic path to the microphone 100.
This configuration presents a couple of issues. First, with the alignment of the apertures in the PWB 104 and the housing 110, what results is essentially a single port. There is a resonance that can form owing to a pocket of air, which acts as a moving mass, that is trapped in that port. That can be undesirable, particularly if the resonance occurs in the frequencies of interest to the user. This can be particularly problematic in products that incorporate a microphone array since the resonance from each microphone in the array may be shifted either in phase or frequency, relative to the other microphones, due to manufacturing differences. That can result in a very complex resonance issue that can be difficult to predict.
The second issue that the arrangement of FIG. 1 presents relates to wind noise. Basically, what this relates to is the amount of surface area that is present along the surface at the opening to the opening to the microphone 100. Where the surface area is very small at the opening to the aperture 112 along the top (outer) surface of the housing 110, there will be higher turbulence, resulting from wind/air passing along the surface, at the opening, which will result is more wind noise. This problem can be exacerbated in products that incorporate a microphone array because there is a cumulative effect since the noise from each microphone in the array contributes to the total noise.
To address/prevent these issues manufacturers have defined very specific dimensional design constraints.